To efficiently separate a solution at low energy and low cost by operating a solution separation device under low pressure.
Reverse osmosis membrane modules 4, 8, and 12 are disposed in a multi-stage manner, wherein the modules have a permeation flux of at least 0.5 m3/m2/MPa/day under a condition that a supply liquid is 0.05 wt.% brine solution, temperature is 25°C, and a pH value is 6.5, and have a salt- inhibition ratio of 99% or more after operation for 30 minutes under such a condition that a supply liquid is 0.05 wt.% brine solution, pressure is 0.5 MPa, temperature is 25°C and a pH value is 6.5. A raw solution is supplied to a first reverse osmosis membrane module 4. A concentrated liquid discharged from the first reverse osmosis membrane module 4 is supplied to a second reverse osmosis membrane module 8. A permeated liquid discharged from the first and second reverse osmosis membrane module 4 and 8 is supplied to a third reverse osmosis membrane module 12.